Mems device

ABSTRACT

A MEMS gyroscope is disclosed herein, wherein the MEMS gyroscope comprised a magnetic sensing mechanism and a magnetic source that is associated with the proof-mass. The magnetic sensing mechanism comprises multiple magnetic field sensors that are designated for sensing the magnetic field from a magnetic source so as to mitigate the problems caused by fabrication.

CROSS-REFERENCE

This US utility patent application claims priority from co-pending US utility patent application “A HYBRID MEMS DEVICE,” Ser. No. 13/559,625 filed Jul. 27, 2012, which claims priority from U.S. provisional patent application “A HYBRID MEMS DEVICE,” filed May 31, 2012, Ser. No. 61/653,408 to Biao Zhang and Tao Ju. This US utility patent application also claims priority from co-pending US utility patent application “A MEMS DEVICE,” Ser. No. 13/854,972 filed Apr. 2, 2013 to the same inventor of this US utility patent application, the subject matter of each of which is incorporated herein by reference in its entirety.

TECHNICAL FIELD OF THE DISCLOSURE

The technical field of the examples to be disclosed in the following sections is related generally to the art of operation of microstructures, and, more particularly, to operation of MEMS devices comprising MEMS magnetic sensing structures.

BACKGROUND OF THE DISCLOSURE

Microstructures, such as microelectromechanical (hereafter MEMS) devices (e.g. accelerometers, DC relay and RF switches, optical cross connects and optical switches, microlenses, reflectors and beam splitters, filters, oscillators and antenna system components, variable capacitors and inductors, switched banks of filters, resonant comb-drives and resonant beams, and micromirror arrays for direct view and projection displays) have many applications in basic signal transduction. For example, a MEMS gyroscope measures angular rate.

A gyroscope (hereafter “gyro” or “gyroscope”) is based on the Coriolis effect as diagrammatically illustrated in FIG. 1. Proof-mass 100 is moving with velocity V_(d). Under external angular velocity Ω, the Coriolis effect causes movement of the poof-mass (100) with velocity V_(s). With fixed V_(d), the external angular velocity can be measured from V_(d). A typical example based on the theory shown in FIG. 1 is capacitive MEMS gyroscope, as diagrammatically illustrated in FIG. 2.

The MEMS gyro is a typical capacitive MEMS gyro, which has been widely studied. Regardless of various structural variations, the capacitive MEMS gyro in FIG. 2 includes the very basic theory based on which all other variations are built. In this typical structure, capacitive MEMS gyro 102 is comprised of proof-mass 100, driving mode 104, and sensing mode 102. The driving mode (104) causes the proof-mass (100) to move in a predefined direction, and such movement is often in a form of resonance vibration. Under external angular rotation, the proof-mass (100) also moves along the V_(s) direction with velocity V_(s). Such movement of V_(s) is detected by the capacitor structure of the sensing mode (102). Both of the driving and sensing modes use capacitive structures, whereas the capacitive structure of the driving mode changes the overlaps of the capacitors, and the capacitive structure of the sensing mode changes the gaps of the capacitors.

Current capacitive MEMS gyros, however, are hard to achieve submicro-g/rtHz because the capacitance between sensing electrodes decreases with the miniaturization of the movable structure of the sensing element and the impact of the stray and parasitic capacitance increase at the same time, even with large and high aspect ratio proof-masses.

Therefore, what is desired is a MEMS device capable of sensing angular velocities and methods of operating the same.

SUMMARY OF THE DISCLOSURE

In view of the foregoing, A MEMS gyroscope is disclosed, the MEMS gyrioscope comprising: a mass-substrate, comprising: a movable prof-mass; and a magnetic source attached to the proof-mass such that the magnetic source is capable of moving with the proof-mass; and a sensor substrate below the mass-substrate, comprising: a magnetic sensing mechanism for detecting a magnet field from the magnetic sensor, wherein the magnetic sensing mechanism is static relative to the magnetic source, wherein the magnetic sensing mechanism further comprising: a plurality of signal sensors associated with said magnetic source for detecting a magnetic field from said magnetic source; and a reference sensor disposed in the vicinity of the signal sensors for detecting a background magnetic signal such that the detections of the signal sensors use the detection of the reference sensor as a reference.

In another example, a wafer assembly is disclosed herein, comprising: a mass wafer comprising a plurality of mass dies, each mass die comprising: a movable prof-mass; and a magnetic source attached to the proof-mass such that the magnetic source is capable of moving with the proof-mass; and a sensor wafer comprising a plurality of sensor dies, each sensor die comprising: a magnetic sensing mechanism for detecting a magnet field from the magnetic sensor, wherein the magnetic sensing mechanism is static relative to the magnetic source, wherein the magnetic sensing mechanism further comprising: a plurality of signal sensors associated with said magnetic source for detecting a magnetic field from said magnetic source; and a reference sensor disposed in the vicinity of the signal sensors for detecting a background magnetic signal such that the detections of the signal sensors use the detection of the reference sensor as a reference.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 diagrammatically illustrates the Coriolis effect in a MEMS structure;

FIG. 2 is a top view of a typical existing capacitive MEMS gyroscope having a driving mode and a sensing mode, wherein both of the driving and sensing mode utilize capacitance structures;

FIG. 3 illustrates an exemplary MEMS gyroscope having a magnetic sensing mechanism;

FIG. 4 illustrates a top view of a portion of an exemplary implementation of the MEMS gyroscope illustrated in FIG. 3, wherein the MEMS gyroscope illustrated in FIG. 4 having a capacitive driving mode and a magnetic sensing mechanism;

FIG. 5 illustrates a perspective view of a portion of another exemplary implementation of the MEMS gyroscope illustrated in FIG. 3, wherein the MEMS gyroscope illustrated in FIG. 5 having a magnetic driving mechanism for the driving mode and a magnetic sensing mechanism for the sensing mode

FIG. 6 illustrates an exemplary magnetic driving mechanism of the MEMS gyroscope in FIG. 5;

FIG. 7 illustrates an exemplary magnetic source of the MEMS gyroscope illustrated in FIG. 3;

FIG. 8 illustrates an exemplary magnetic sensing mechanism that can be used in the MEMS gyroscope illustrated in FIG. 3;

FIG. 9 shows an exemplary thin-film stack that can be configured into a CIP or CPP structure for use in the magnetic sensing mechanism illustrated in FIG. 8;

FIG. 10 illustrates an exemplary MEMS gyroscope that comprises multiple magnetic sensing structures;

FIG. 11 illustrates an exemplary operation for detecting and measuring an angular velocity using a MEMS gyroscope illustrated in FIG. 3;

FIG. 12 illustrates temperature dependence of the coercivity of a ferromagnetic thin film, wherein the ferromagnetic thin film can be used in the signal sensor illustrated in FIG. 3;

FIG. 13 illustrates temperature dependence of the coercivity of a ferromagnetic thin film, wherein the ferromagnetic thin film can be used in the signal sensor illustrated in FIG. 3;

FIG. 14 illustrates the temperature dependence of the magnetic exchange field between a pining layer and a free layer, wherein the pinning layer and the free layer can be used in the signal sensor illustrated in FIG. 3;

FIG. 15 illustrates the top view of an exemplary sensor wafer that comprises multiple magnetic sensors, wherein the multiple magnetic sensors are associated with one magnetic source of the proof-mass and are designated for measuring the magnetic field from said magnetic source;

FIG. 16 illustrates a mass-wafer and a magnetic sensor wafer, each having multiple dies;

FIG. 17 illustrates a wafer assembly comprising the mass-wafer and magnetic sensor wafer of FIG. 16; and

FIG. 18 illustrates a side view of the wafer assembly of FIG. 17.

DETAILED DESCRIPTION OF SELECTED EXAMPLES

Disclosed herein is a MEMS gyroscope and method of using the same for sensing an angular velocity, wherein the MEMS gyroscope utilizes a magnetic sensing mechanism. It will be appreciated by those skilled in the art that the following discussion is for demonstration purposes, and should not be interpreted as a limitation. Many other variations within the scope of the following disclosure are also applicable. For example, the MEMS gyroscope and the method disclosed in the following are applicable for use in accelerometers.

Referring to FIG. 3, an exemplary MEMS gyroscope is illustrated herein. In this example, MEMS gyroscope 106 comprises magnetic sensing mechanism 114 for sensing the target angular velocity through the measurement of proof-mass 112. Specifically, MEMS gyroscope 106 comprises mass-substrate 108 and sensor substrate 110. Mass-substrate 108 comprises proof-mass 112 that is capable of responding to an angular velocity. The two substrates (108 and 110) are spaced apart, for example, by a pillar (not shown herein for simplicity) such that at least the proof-mass (112) is movable in response to an angular velocity under the Coriolis effect. The movement of the proof-mass (112) and thus the target angular velocity can be measured by magnetic sensing mechanism 114.

The magnetic sensing mechanism (114) in this example comprises a magnetic source 116 and magnetic sensor 118. The magnetic source (116) generates a magnetic field, and the magnetic sensor (118) detects the magnetic field and/or the magnetic field variations that is generated by the magnetic source (116). In the example illustrated herein in FIG. 3, the magnetic source is placed on/in the proof-mass (112) and moves with the proof-mass (112). The magnetic sensor (118) is placed on/in the sensor substrate (120) and non-movable relative to the moving proof-mass (112) and the magnetic source (116). With this configuration, the movement of the proof-mass (112) can be measured from the measurement of the magnetic field from the magnetic source (116).

Other than placing the magnetic source on/in the movable proof-mass (1112), the magnetic source (116) can be placed on / in the sensor substrate (120); and the magnetic sensor (118) can be placed on/in the proof-mass (112).

It is also noted that the MEMS gyroscope illustrated in FIG. 3 can also be used as an accelerometer.

The MEMS gyroscope as discussed above with reference to FIG. 3 can be implemented in many ways, one of which is illustrated in FIG. 4. Referring to FIG. 4, the proof-mass (120) is driven by capacitive, such as capacitive comb. The sensing mode, however, is performed using the magnetic sensing mechanism illustrated in FIG. 3. For this reason, capacitive combs can be absent from the proof-mass (120).

Alternatively, the proof-mass can be driven by magnetic force, an example of which is illustrated in FIG. 5. Referring to FIG. 5, the mass substrate (108) comprises a movable proof-mass (126) that is supported by flexible structures such as flexures 128, 129, and 130. The layout of the flexures enables the proof-mass to move in a plane substantially parallel to the major planes of mass substrate 108. In particular, the flexures enables the proof-mass to move along the length and the width directions, wherein the length direction can be the driving mode direction and the width direction can be the sensing mode direction of the MEMS gyro device. The proof-mass (126) is connected to frame 132 through flexures (128, 129, and 130). The frame (132) is anchored by non-movable structures such as pillar 134. The mass-substrate (108) and sensing substrate 110 are spaced apart by the pillar (134). The proof-mass (112) in this example is driving by a magnetic driving mechanism (136). Specifically, the proof-mass (126) can move (e.g. vibrate) in the driving mode under magnetic force applied by magnetic driving mechanism 136, which is better illustrated in FIG. 6.

Referring to FIG. 6, the magnetic driving mechanism 136 comprise a magnet core 138 surrounded by coil 140. By applying an alternating current through coil 140, an alternating magnetic field can be generated from the coil 140. The alternating magnetic field applies magnetic force to the magnet core 140 so as to move the magnet core. The magnet core thus moves the proof-mass.

The magnetic source (114) of the MEMS gyroscope (106) illustrated in FIG. 3 can be implemented in many ways, one of which is illustrated in FIG. 7. Referring to FIG. 7, conductive wire 142 is displaced on/in proof-mass 112. In one example, conductive wire 142 can be placed on the lower surface of the proof-mass (112), wherein the lower surface is facing the magnetic sensors (118 in FIG. 3) on the sensor substrate (110, in FIG. 3). Alternatively, the conductive wire (142) can be placed on the top surface of the proof-mass (112), i.e. on the opposite side of the proof-mass (112) in view of the magnetic sensor (118). In another example, the conductive wire (142) can be placed inside the proof-mass, e.g. laminated or embedded inside the proof-mass (112), which will not be detailed herein as those examples are obvious to those skilled in the art of the related technical field.

The conductive wire (142) can be implemented in many suitable ways, one of which is illustrated in FIG. 7. In this example, the conductive wire (142) comprises a center conductive segment 146 and tapered contacts 144 and 148 that extend the central conductive segment to terminals, through the terminals of which current can be driven through the central segment. The conductive wire (142) may have other configurations. For example, the contact tapered contacts (144 and 148) and the central segment (146) maybe U-shaped such that the tapered contacts may be substantially parallel but are substantially perpendicular to the central segment, which is not shown for its obviousness.

The magnetic sensor (118) illustrated in FIG. 3 can be implemented to comprise a reference sensor (150) and a signal sensor (152) as illustrated in FIG. 8. Referring to FIG. 8, magnetic senor 118 on/in sensor substrate 120 comprises reference sensor 150 and signal sensor 152. The reference sensor (150) can be designated for dynamically measuring the magnetic signal background in which the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7) co-exists. The signal sensor (152) can be designated for dynamically measuring the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7). In other examples, the signal sensor (152) can be designated for dynamically measuring the magnetic signal background in which the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7) co-exists, while the signal sensor (150) can be designated for dynamically measuring the target magnetic signal (e.g. the magnetic field from the conductive wire 146 as illustrated in FIG. 7).

The reference sensor (150) and the signal sensor (152) preferably comprise magneto-resistors, such as AMRs, giant-magneto-resistors (such as spin-valves, hereafter SV), or tunneling-magneto-resistors (TMR). For demonstration purpose, FIG. 9 illustrates a magneto-resistor structure, which can be configured into CIP (current-in-plane, such as a spin-valve) or a CPP (current-perpendicular-to-plane, such as TMR structure). As illustrated in FIG. 9, the magneto-resistor stack comprises top pin-layer 154, free-layer 156, spacer 158, reference layer 160, bottom pin layer 162, and substrate 120. Top pin layer 154 is provided for magnetically pinning free layer 156. The top pin layer can be comprised of IrMn, PtMn or other suitable magnetic materials. The free layer (156) can be comprised of a ferromagnetic material, such as NiFe, CoFe, CoFeB, or other suitbale materials or the combinations thereof. The spacer (158) can be comprised of a non-magnetic conductive material, such as Cu, or an oxide material, such as Al₂O₃ or MgO or other suitable materials. The reference layer (160) can be comprised of a ferromagnetic magnetic material, such as NiFe, CoFe, CoFeB, or other materials or the combinations thereof. The bottom pin layer (162) is provided for magnetic pinning the reference layer (160), which can be comprised of a IrMn, PtMn or other suitable materials or the combinations thereof. The substrate (120) can be comprised of any suitable materials, such as glass, silicon, or other materials or the combinations thereof.

In examples wherein the spacer (158) is comprised of a non-magnetic conductive layer, such as Cu, the magneto-resistor (118) stack can be configured into a CIP structure (i.e. spin-valve, SV), wherein the current is driven in the plane of the stack layers. When the spacer (158) is comprised of an oxide such as Al₂O₃, MgO or the like, the magneto-resistor stack (118) can be configured into a CPP structure (i.e. TMR), wherein the current is driven perpendicularly to the stack layers.

In the example as illustrated in FIG. 9, the free layer (156) is magnetically pinned by the top pin layer (154), and the reference layer (160) is also magnetically pinned by bottom pin layer 162. The top pin layer (154) and the bottom pin layer (162) preferably having different blocking temperatures. In this specification, a blocking temperature is referred to as the temperature, above which the magnetic pin layer is magnetically decoupled with the associated pinned magnetic layer. For example, the top pin layer (154) is magnetically decoupled with the free layer (156) above the blocking temperature T_(B) of the top pin layer (154) such that the free layer (156) is “freed” from the magnetic pinning of top pin layer (154). Equal to or below the blocking temperature T_(B) of the top pin layer (154), the free layer (156) is magnetically pinned by the top pin layer (154) such that the magnetic orientation of the free layer (156) is substantially not affected by the external magnetic field. Similarly, the bottom pin layer (162) is magnetically decoupled with the reference layer (160) above the blocking temperature T_(B) of the bottom pin layer (162) such that the reference layer (160) is “freed” from the magnetic pinning of bottom pin layer (162). Equal to or below the blocking temperature T_(B) of the bottom pin layer (162), the reference layer (160) is magnetically pinned by the bottom pin layer (162) such that the magnetic orientation of the reference layer (162) is substantially not affected by the external magnetic field.

The top and bottom pin layers (154 and 162, respectively) preferably have different blocking temperatures. When the free layer (156) is “freed” from being pinned by the top pin layer (154), the reference layer (160) preferably remains being pinned by the bottom pin layer (162). Alternatively, when the free layer (156) is still pinned by the top pin layer (154), the reference layer (160) can be “freed” from being pinned by the bottom pin layer (162). In the later example, the reference layer (160) can be used as a “sensing layer” for responding to the external magnetic field such as the target magnetic field, while the free layer (156) is used as a reference layer to provide a reference magnetic orientation.

The different blocking temperatures can be accomplished by using different magnetic materials for the top pin layer (154) and bottom pin layer (162). In one example, the top pin layer (154) can be comprised of IrMn, while the bottom pin layer (162) can be comprised of PtMn, vice versa. In another example, both of the top and bottom pin layers (154 and 162) may be comprised of the same material, such as IrMn or PtMn, but with different thicknesses such that they have different blocking temperatures.

It is noted by those skilled in the art that the magneto-resistor stack (118) is configured into sensors for sensing magnetic signals. As such, the magnetic orientations of the free layer (156) and the reference layer (160) are substantially perpendicular at the initial state. Other layers, such as protective layer Ta, seed layers for growing the stack layers on substrate 120 can be provided. It is further noted that the magnetic stack layers (118) illustrated in FIG. 9 are what is often referred to as “bottom pin” configuration in the field of art. In other examples, the stack can be configured into what is often referred as “top pinned” configuration in the field of art, which will not be detailed herein.

In some applications, multiple magnetic sensing mechanisms can be provided, an example of which is illustrated in FIG. 10. Referring to FIG. 10, magnetic sensing mechanisms 116 and 164 are provided for detecting the movements of proof-mass 112. The multiple magnetic sensing mechanisms can be used for detecting the movements of proof-mass 112 in driving mode and sensing mode respectively. Alternatively, the multiple magnetic sensing mechanisms 116 and 164 can be provided for detecting the same modes (e.g. the driving mode and/or the sensing mode).

By using the different blocking temperatures of the sensors as discussed above with reference to FIG. 9, the reference sensor (150) and signal sensor (152) can be dynamically activated or deactivated for sensing the target magnetic field. For demonstration purpose, FIG. 11 shows an exemplary operation method of measuring a target magnetic field (e.g. from the wire 146 as illustrated in FIG. 7) by using the magnetic sensor (118 as illustrated in FIG. 8).

With reference to FIG. 7, FIG. 8, and FIG. 11, the wire can be set to the OFF state by not driving a current through the wire at the initial time T_(o). The reference sensor can be set to the ON state. The ON state of the reference sensor can be achieved through “freeing” the free layer of the reference sensor by raising the temperature of the pin layer that pins the free layer of the reference sensor above its blocking temperature (e.g. by applying a series of heating pulses or current pulses) and driving a current through the reference sensor so as to measure its magneto-resistance. The signal sensor at this time can be set to the ON or any other suitable state, even though it is preferred that the signal sensor can be set to the OFF state to avoid the magnetic field generated by the current driven through the signal sensor for setting the signal sensor to the ON state. Because the wire is set to the OFF state and no current is driven through the wire, the reference sensor measures the instant magnetic signal background at time T_(o). After the reference sensor finished the measurement, it locks its instant state at time T₁ by for example, lowering the temperature of its top pin layer (used for pinning the free layer) below its blocking temperature such that the free layer is magnetically coupled to (thus pinned by) the top pin layer. The reference sensor at this state is referred to as the “Lock” state.

At time T₁, the wire remains OFF and the signal sensor can be at any state. When the reference sensor is stabled at the “Lock” state (e.g. finishes “locking” the state of its free layer), the wire is set to the ON state at time T₂, by driving current with pre-defined amplitude through the wire so as to generate magnetic field. The current can be DC or AC. After the magnetic field generated by the wire is stabilized, the signal sensor can be set to the ON state. Setting the signal sensor to the ON state can be accomplished by raising the temperature of the pin layer used for pinning the free layer of the signal sensor above its blocking temperature so as to free the free layer. A current is driven through the signal sensor so as to measure its magneto-resistance.

After the signal sensor finished the measurement, it locks its instant state at time T₃ by for example, lowering the temperature of its top pin layer (used for pinning the free layer) below its blocking temperature such that the free layer is magnetically coupled to (thus pinned by) the top pin layer. The signal sensor at this state is referred to as the “Lock” state.

When the signal sensor finishes its locking at time T₃, the reference sensor and the signal sensor can output their measurements to so to obtain the magnetic field from the magnetic source attached to the proof-mass, thus extract the information of the movement of the proof-mass.

The reference sensor and the signal sensor can be connected by a Wheatstone bridge, or can be connected directly to an amplifier or other electrical circuits to obtain the target magnetic field, which not be detailed herein.

In the example discussed above, the reference sensor and/or the signal sensor can be configured to “lock” the status (e.g. the detected magnetic signal). This locking capability can be accomplished in many ways. In the following, such locking capability will be discussed with reference to signal sensor, and the reference sensor can be implemented in substantially the same ways.

In one example, the signal sensor can be configured to be comprised of a storage layer that comprises a ferromagnetic layer. The storage layer is connected to electrical leads such that electrical current can be applied through the storage layer. When current is applied, the storage layer is heated, and its temperature can be elevated.

The material, as well as the geometry (e.g. the thickness) of the storage layer can be configured such that at the elevated temperature above a threshold temperature, such as the Currie temperature, the storage layer is capable of being magnetized by the target magnetic signal so as to accomplish the detection of the target magnetic signal. When the temperature of the storage layer is dropped to a temperature below the threshold temperature, the storage layer “freezes” its magnetization states so as to accomplish its “locking” operation. FIG. 12 illustrates such operation.

Referring to FIG. 12, the vertical axis plots the coercivity of the storage layer (e.g. the ferromagnetic layer of the storage layer); and the horizontal axis plots the temperature. The coercivity of the storage layer (ferromagnetic layer) decreases with increased temperature. At room temperature RT, the storage layer has a coercivity that is higher than the target magnetic signal H_(target), therefore, the storage layer is unable to detect the target magnetic signal. As the temperature of the storage layer increases, the coercivity of the storage layer decreases. At the storing temperature (or the blocking temperature wherein the signal storage layer transits from ferromagnetic to paramagnetic or super-paramagnetic), the coercivity of the storage layer is equal to or less than the target magnetic signal H_(signal) such that the storage layer is capable of being magnetized and thus detecting the target magnetic signal. After the detection, the temperature of the storage layer can be decreased, by for example, removing the current applied through the storage layer (e.g. the ferromagnetic layer of the storage layer). When the temperature of the storage layer is decreased to a temperature below the storing temperature, the magnetization state of the ferromagnetic layer (storage layer) is “locked” because the coercivity of the storage layer is higher than the target magnetic field. With this mechanism, the storage layer accomplishes the “locking process.”

The coercivity of a magnetic thin-film (layer) also varies with its thickness, as diagrammatically illustrated in FIG. 13. The storage layer can have a thickness such that the coercivity of the signal-storage layer is in the vicinity of the target magnetic field H_(target), such as within a range of ±0.5%, ±1%, ±1.5%, ±2%, ±2.5%, ±3%, ±4%, ±5%, ±8%, ±10% of H_(target). Especially when the storage layer has a thickness such that its coercivity is higher than H_(target), a thermal layer can be provided to adjust the coercivity of the storage layer.

In addition to utilizing the temperature dependence of coercivity of a ferromagnetic layer, a magnetic coupling structure can be utilized to accomplish the “locking” process, as illustrated in FIG. 14. Referring to FIG. 4, the signal sensor comprises free layer 172 and pinning layer 170. The free layer (172) is a ferromagnetic layer, and is provide for responding to the target magnetic field to be detected. The pinning layer (170) is an antiferromagnetic layer, and is provided for magnetically pining the free layer (172) through the exchange magnetic field H_(exch). It is known in the art that the magnetic exchange field H_(exch) changes with temperature. When the temperature is higher than the blocking temperature T_(B) that characterizes the magnetic exchange field between the free layer (172) and pining layer (170), the magnetic exchange field between the free layer (172) and pining layer (172) is broken, e.g. reduced to a level such that the free layer (172) and pinning layer (172) are magnetically decoupled. The free layer (172) is not pinned by the pinning layer (172) at this temperature. By utilizing such magnetic-couple (pinning) and magnetic-decouple (unpinning), the signal sensor comprising the free layer (172) and pining layer (170) can accomplish the state “locking” process.

For example, when it is desired to detect the target magnetic field signal, the signal sensor may elevate its temperature above the blocking temperature T_(B) by for example, applying current through the free layer (172) and/or the pining layer (170). The free layer (172) is thus “freed” and can be used for picking up the target magnetic field signal. When it is desired for the signal sensor to lock its detection, for example, after the detecting the target magnetic signal, the signal sensor can decrease its temperature below the blocking temperature T_(B). At a temperature below T_(B), the free layer (172) is magnetically pinned by the pinning layer (172). The magnetic states of the free layer (172), which corresponds to the target magnetic field signal, is thus “frozen” in the free layer (172).

In the above discussion, a magnetic sensing mechanism is provided for detecting the magnetic field from the magnetic source of the proof-mass. However, the magnetic source and the magnetic sensors are in different substrates, such as the mass-substrate 108 and sensor substrate 110 as shown in FIG. 3. During fabrication, the mass-substrate (108) and sensor substrate (110) are aligned and assembled into a wafer assembly. One of the requirement of the wafer alignment is that the magnetic source (116) of the mass-substrate (108) is expected to be aligned to the magnetic sensor (118) according to a pre-determined relative geometric positions. In one example as illustrated in FIG. 3, the geometric center of active area of the magnetic source (116) (e.g. the geometric center of the conducting wire segment as illustrated in FIG. 7) is aligned to the geometric center of the active area of the magnetic sensor (118) when view from the top. In other examples, the two geometric centers can be offset by a pre-determined distance when viewed from the top, especially when the offset position offers better magnetic field gradient, which will not be detailed herein. In some other examples, the relative position of the magnetic source and magnetic sensor can be arranged so as to obtaining better magnetic field signal strength.

Regardless of various relative position arrangements, the mass-substrate (108) and sensor substrate 110 (as illustrated in FIG. 3) are expected to be aligned according to a predetermined scheme. In fabrication, such expectation however is not always guaranteed. Alignment error occurs oftentimes during the process of assembling the mass-substrate (108) and sensor substrate (110). Such misalignment can be of detrimental because the magnetic field from the magnetic source decreases with the squared distance. The magnetic sensor (118) may not be able to detects the magnetic field from the associated magnetic sensor (116) if the misalignment too large such that the strength of the magnetic field from the magnetic source (116) at the location of the associated magnetic sensor (118) is smaller than the sensitivity of the magnetic sensor (118), or the magnetic field gradient of the magnetic field from the magnetic source (116) is too small to be detected by the magnetic sensor (118).

The above problem caused by wafer misalignment can be solved by increasing the accuracy of the alignment during the wafer assembly, which however, is extremely hard with present fabrication technologies. This disclosure provides an alternative approach to remedy the misalignment. Referring to FIG. 15, multiple magnetic sensors are provided at different locations such as locations A, B, C. These locations are determined by at least the alignment accuracy during the wafer assembly. For example, when the alignment accuracy of the wafer assembly is ΔD (the accuracy along one direction), the distance between adjacent locations such as between A, B, and C can be αΔD, wherein α is a constant, preferably between 0.1 to 5, and more preferably, between 0.5 to 2, and more preferably between 1 and 2, such as 1.5. In some examples, the distance between the adjacent locations A, B, and C can also include the factor of the magnetic field gradient of the magnetic field from the magnetic source.

With the multiple magnetic sensors provided at the pre-determined different locations, such as A, B, and C, the problem caused by the misalignment during the wafer assembly can be mitigated, if not eliminated. For example, the magnetic source (146) is expected to be at location A and associated with magnetic sensor 118. After the wafer assembling, the magnetic source may be in the vicinity of position B or C due to assembling misalignment. Magnetic sensor 118 in this situation may not be optimal for detecting the magnetic field from the magnetic source 146. However, magnetic sensor 182 (for position B) or 184 (for position C) can be used for effectively detecting the magnetic field from the magnetic source (146). In this way, the problem caused by misalignment during wafer assembling can be mitigated or eliminated.

The magnetic sensors (118, 182, and 184) each have a reference sensor and a signal sensor as discussed in the above sections. Alternatively, the magnetic sensors each can be comprised of a signal sensor, such as those discussed above with reference to FIG. 8. The magnetic sensor can be a GMR (giant magneto-resistor), a spin-valve, a MTJ (magnetic tunnel junction), a Hall sensor, or other types of magnetic sensors capable of sensing magnetic field, or a combination thereof if necessary.

In another example, a reference sensor can be provided for multiple signal sensors, as illustrated in FIG. 15. As shown in FIG. 15, magnetic sensors 118, 150 and 184 are signal sensors designated for detecting the magnetic field from the magnetic source of the proof-mass. Reference sensor 152 is designated for detecting the background magnetic signals. Detections from the signal sensors 118, 150, and 184 all may use the detection from reference sensor 152 as a reference signal for comparison. In this example, multiple signals sensors can share a single reference sensor for detection.

The reference sensor (152) can be placed at any suitable locations within an area in which the associated signal sensors are disposed. For example, reference sensor 152 can be placed in the vicinity of signal sensor 118, 150, or 184. However, it is preferred that the reference sensor (152) is at a location that is as close to the geometric center of the associated signal sensors as possible. Alternatively, the reference sensor (152) can be at a location that is as close to a symmetric center to which the associated signal sensors are substantially symmetric. By such configuration, the reference sensor (152) is capable of providing a substantially “uniform reference signal” to all associated signal sensors. If the reference sensor (152) is disposed too far away such that the reference signal provided by such reference sensor (152) may not be suitable for all signal sensors.

The MEMS gyroscope as discussed above can be fabricated on a wafer level. For example, as illustrated in FIG. 16. Wafer 186 can be of any size, such as 2 inch, 4 inch, 6 inch, 8 inch, 16 inch, and it can be of any shape, such as circular with a notch, rectangular, square. Wafer 186 is comprised of a material suitable for fabricating the MEMS features, such as the proof-mass, flextures, frames in the mass-substrate (e.g. mass-substrate 108 illustrated in FIG. 3). A plurality of dies, such as die 188, is formed in wafer 186. The MEMS features in the mass-substrate are formed in each die.

Wafer 190 is a magnetic sensor wafer with a plurality of magnetic sensing mechanisms formed thereon. Wafer 190 can be of any size and shape as desired as wafer 186. Wafer 190 is comprised of a material that is suitable for forming the desired magnetic sensors. Multiple dies are formed in wafer 190, such as die 192. And each die comprises a desired magnetic sensing mechanism as discussed in above sections.

The mass-wafer (186) and the magnetic sensor wafer 190 are assembled into a wafer assembly as illustrated in FIG. 17 and FIG. 18. The mass wafer (186) and the magnetic sensor wafer 190 are assembled into wafer assembly 194. The dies in the mass wafer (186) and magnetic sensor wafer 190 are assembled into die assemblies, such as die assembly 196. The wafer assembly may comprise other features, such as saw lines for dicing. The wafer assembly (194) can further comprise packages that are disposed through a wafer level packaging, which will not be detailed herein. The wafer assembly 194 is better illustrated in a side view, as illustrated in FIG. 18.

Referring to FIG. 18, wafer assembly 194 comprises mass wafer 186 and magnetic sensor wafer 190. The mass wafer (186) comprises multiple dies, such as die 188 having MEMS features formed thereon. The magnetic sensor wafer (190) comprises multiple dies such as die 192 having a magnetic sensing mechanism formed thereon. The dies 188 and 192 are aligned such that, at least the magnetic source of the proof-mass is aligned to the associated magnetic sensor. The dies 188 and 192 are assembled into a die assembly 196. The die assembly can be separated from the wafer assembly 194 so as to form a desired MEMS gyroscope device.

It will be appreciated by those of skilled in the art that a new and useful MEMS gyroscope has been described herein. In view of the many possible embodiments, however, it should be recognized that the embodiments described herein with respect to the drawing figures are meant to be illustrative only and should not be taken as limiting the scope of what is claimed. Those of skill in the art will recognize that the illustrated embodiments can be modified in arrangement and detail. Therefore, the devices and methods as described herein contemplate all such embodiments as may come within the scope of the following claims and equivalents thereof. In the claims, only elements denoted by the words “means for” are intended to be interpreted as means plus function claims under 35 U.S.C. §112, the sixth paragraph. 

We claim:
 1. A MEMS gyroscope, comprising: a mass-substrate, comprising: a movable prof-mass; and a magnetic source attached to the proof-mass such that the magnetic source is capable of moving with the proof-mass; and a sensor substrate below the mass-substrate, comprising: a magnetic sensing mechanism for detecting a magnet field from the magnetic sensor, wherein the magnetic sensing mechanism is static relative to the magnetic source, wherein the magnetic sensing mechanism further comprising: a plurality of signal sensors associated with said magnetic source for detecting a magnetic field from said magnetic source; and a reference sensor disposed in the vicinity of the signal sensors for detecting a background magnetic signal such that the detections of the signal sensors use the detection of the reference sensor as a reference.
 2. The MEMS gyroscope of claim 1, wherein the magnetic source comprises a conductive wire to which current can be applied so as to generate a magnetic field.
 3. The MEMS gyroscope of claim 1, wherein the magnetic source comprises a magnetic nanoparticle.
 4. The MEMS gyroscope of claim 2, wherein conductive wire has a length along a length of at least one of the plurality of magnetic sensing mechanisms.
 5. The MEMS gyroscope of claim 2, wherein at least one of the plurality of magnetic sensors comprises a giant-magnetic-resistor.
 6. The MEMS gyroscope of claim 2, wherein at least one of the plurality of magnetic sensors comprises a spin-valve structure.
 7. The MEMS gyroscope of claim 2, wherein at least one of the plurality of magnetic sensors comprises a tunnel-magnetic-resistor.
 8. The MEMS gyroscope of claim 2, wherein at least one of the magnetic sensors comprises magnetic pickup coil that is an element of a fluxgate.
 9. The MEMS gyroscope of claim 2, wherein the magnetic sensors each has a geometric length and a width, wherein the geometric lengths of the magnetic sensors are substantially parallel, and are substantially parallel to the length of the conductive wire.
 10. The MEMS gyroscope of claim 2, wherein at least one of the magnetic sensors comprises a reference sensor and a signal sensor pair.
 11. A wafer assembly, comprising: a mass wafer comprising a plurality of mass dies, each mass die comprising a movable prof-mass; and a magnetic source attached to the proof-mass such that the magnetic source is capable of moving with the proof-mass; and a sensor wafer comprising a plurality of sensor dies, each sensor die comprising: a magnetic sensing mechanism for detecting a magnet field from the magnetic sensor, wherein the magnetic sensing mechanism is static relative to the magnetic source, wherein the magnetic sensing mechanism further comprising: a plurality of signal sensors associated with said magnetic source for detecting a magnetic field from said magnetic source; and a reference sensor disposed in the vicinity of the signal sensors for detecting a background magnetic signal such that the detections of the signal sensors use the detection of the reference sensor as a reference.
 12. The wafer assembly of claim 11, wherein the magnetic source comprises a conductive wire to which current can be applied so as to generate a magnetic field.
 13. The wafer assembly of claim 11, wherein the magnetic source comprises a magnetic nanoparticle.
 14. The wafer assembly of claim 12, wherein conductive wire has a length along a length of at least one of the plurality of magnetic sensing mechanisms.
 15. The wafer assembly of claim 12, wherein at least one of the plurality of magnetic sensors comprises a giant-magnetic-resistor.
 16. The wafer assembly of claim 12, wherein at least one of the plurality of magnetic sensors comprises a spin-valve structure.
 17. The wafer assembly of claim 12, wherein at least one of the plurality of magnetic sensors comprises a tunnel-magnetic-resistor.
 18. The wafer assembly of claim 12, wherein at least one of the magnetic sensors comprises magnetic pickup coil that is an element of a fluxgate.
 19. The wafer assembly of claim 12, wherein the magnetic sensors each has a geometric length and a width, wherein the geometric lengths of the magnetic sensors are substantially parallel, and are substantially parallel to the length of the conductive wire.
 20. The wafer assembly of claim 12, wherein at least one of the magnetic sensors comprises a reference sensor and a signal sensor pair. 